Impedance translating device



April 26, R ADLER IMPEDANCE TRANSLATING DEVICE Filed Nov. 3, 1949Wa/XVENTOR. V

BY 4 JMW2% W United States Patent 0 IMPEDANCE TRAN SLATING DEVICE RobertAdler, Chicago, Ill., assignor to Consolidated Electric Company,Chicago, 111., a corporation of Illinois Application November 3, 1949,Serial No. 125,249 8 Claims. (Cl. 250-36) This invention relates to anaudio and lower frequency impedance translating device and in particularas applied to a variable frequency oscillator, as applied to a variablefrequency resonant circuit, and simply as an impedance device having anapparent impedance of adjustable value, the apparent impedance for anygiven adjustment remaining constant over a wide range of voltages orfrequencies of an external voltage applied to said device. It is anobject of this invention to provide improved apparatus of thatcharacter.

It is well known to obtain variable capacitance through the use ofrotatable condenser plates or through the use of multiple condensershaving selectable taps. Where it is necessary that space requirements bekept at a minimum, the first of these possibilities is suitable only forhigher frequencies, i. e.,small capacities, because, at audio and lowerfrequencies generally, the condensers become large due to the need forlarge capacities and the large spacing between plates. The second ofthese arrangements is not completely satisfactory because the capacityis variable only in finite steps and because of variable contactresistance. Furthermore, either of these arrangements requires asubstantial operating force because of unavoidable, attendant frictionand thus is useless where relatively small mechanical forces areavailable.

Fixed condensers can be made sufficiently small because metal foil typesmay be used and these can be made with properties substantiallyindependent of variables such as temperature, voltage, etc.

It is well known to obtain variable inductance through the use of slidewire devices, multiple tap devices, coils in which a core is slidable,and variometers, the latter being a transformer having an air gap with acoil rotatably mounted therein.

Here, at high frequencies, air core coils can be used because the valuesof inductance needed are small, and since the permeability of air isconstant variable inductances can be made stable, that is, theinductance does not change substantially with different voltages undervarying temperature conditions, etc. Where the needed inductance valuesare higher, powdered metal cores have been used in variable inductancesat high frequencies with good stability.

At audio frequencies and lower frequencies generally, it is necessary toutilize iron core devices of some sort to obtain the required values ofinductance. Whether such audio frequency inductance is of the variometertype or any other one of the types indicated, inductance is proportionalto the permeability of the iron, and since the permeability of iron is afunction of the applied voltage, known variable inductance devices foraudio frequencies are not stable.

Slide wire devices are unsuitable because of relatively high requiredoperating force in applications where only small forces are available,and where continuous variations in inductance are needed multiplecontact devices obviously are not satisfactory.

Fixed inductance devices can be made sufiiciently stable through the useof good iron and a well designed air gap.

Tuned circuits or oscillators of variable frequency in which such knownvariable reactance devices are employed are limited in their applicationor usefulness because of the limitations discussed above in theadjustability and stability of the impedance devices.

Conventional variable resistances such as carbon piles, slide wireresistors and tapped resistors also have various disadvantages such asinstability, and variable contact resistance.

It is an object of the invention to provide an improved variableimpedance which is stable over the desired range of voltage, frequency,temperature and other common variables.

While the invention is particularly applicable to alternating currentsof relatively low frequencies, such as those within and below the rangetermed audio frequencies, it may be applied to circuits utilizing higherfrequencies.

According to the invention, an impedance device of adjustable apparentor effective value is obtained by placing in series with a fixedimpedance element having a stable impedance value an internal voltagewhich may buck or boost an external voltage applied to the device. Thisinternal voltage is of the same character as the external or appliedvoltages, that is, if the external voltage is alternating, the internalvoltage must be an alternating voltage of the same frequency and inproper phase relation therewith. Under these circumstances, as willsubsequently be explained in greater detail, the current flow resultingfrom the application of an external voltage will be such as to indicatean impedance of greater or lesser value than the actual impedance of thefixed impedance element, depending upon the relative polarity or phaserelationship of the internal voltage with respect to the appliedvoltage. The apparent impedance of the device can be varied byselectably adjusting the magnitude of the internal voltage in eitherbucking or boosting relationship with respect to the external or appliedvoltage.

Where the applied voltage is of variable magnitude, the internalvoltage, in addition to being of the same character as the appliedvoltage, must be varied in magnitude in direct proportion to theexternal voltage in order to maintain a constant apparent impedance fora given setting or adjustment of the device. This may be accomplished byhaving the character and magnitude of the internal voltage derived fromthe applied voltage. The circuit is preferably arranged to isolate theapplied voltage, as through the use of an electron amplifier tubecircuit, since any current drawn from the source of applied voltage willcause a change in the apparent impedance of the device.

According to one embodiment of the invention, a selectable internalvoltage is obtained from a rotary transformer which is continuouslyexcited by the applied voltage through an electron tube circuit. Thisarrangement results in the character and magnitude of the internalvoltage being derived from the applied voltage while calling for anegligible fiow of current from the source of applied voltage.

For any given applied voltage the internal voltage may be varied over aiven range by adjustment of the rotary transformer. The range ofapparent impedance thereby obtained centers about the constant impedanceof the fixed impedance element. This range may be increased or decreasedby varying the gain of the isolating amplifier which excites the rotarytransformer, the increased or decreased range of selectable impedancevalues still being centered about the same central value. This featureis of great practical value in bringing the device into properadjustment.

The resonant frequency of a tuned circuit may be varied by utilizingsuch a variable reactance device therein, that is, a voltage is placedin series with either the capacitive or the inductive reactance of theresonant circuit, the value of this voltage being automaticallymaintained at some desired fraction of the voltage applied to the tunedcircuit. The resonant frequency of such a circuit may be varied smoothlyover a Wide range, and with any selected setting will remain constanteven though the voltage applied thereto varies in magnitude.

The controlling or frequency determining portion of one type ofoscillator is a resonant circuit comprising at least one condenser andone inductance coil. The output frequency of such an oscillator may bemade selectable by utilizing a resonant circuit such as that describedimmediately above, in which case the output frequency may be smoothlyadjusted and, for any selected setting, will remain constant even thoughthe voltage or current output of the oscillator varies over a widerange.

finite steps of variability,

Accordingly, it is another object of the invention to provide animproved oscillator having a smoothly variable and selectable outputfrequency.

It is another object of the invention to provide an improved oscillatorhaving an output frequency which is smoothly and selectably variable andwhich remains substantially constant for a given setting in spite ofsubstantial variation of the voltage or current output thereof.

It is another object of the invention to provide an improved resonantcircuit of smoothly variable and selectable resonant frequency.

It is another object of the invention to provide an improved resonantcircuit having a resonant frequency which is smoothly and selectablyvariable and which remains substantially constant at any given settingin spite of substantial variation of the voltage applied thereto.

It is another object of the invention to provide an improved impedancedevice of smoothly variable and selectable apparent impedance.

It is another object of the invention to provide an improved impedancedevice of smoothly variable and selectable apparent impedance, suchapparent impedance remaining substantially constant for any givensetting of said device in spite of substantial variation of themagnitude of the voltage applied thereto.

It is another object of the invention to provide an improved impedancedevice having an apparent impedance which is smoothly variable andselectable over a given range, that range being smoothly and selectablyvariable.

It is another object of the invention to provide an improved impedancedevice having an apparent impedance which is smoothly variable andselectable over a given range, that range being smoothly and selectablyvariable about the same central value of impedance.

It is another object of the invention to provide an improved reactancedevice having a smoothly variable and selectable apparent reactance.

It is another object of the invention to provide an improved reactancedevice having a smoothly variable and selectable reactance, suchapparent reactance remaining constant for a given setting of said devicein spite of substantial variation of the magnitude or frequency of analternating voltage applied thereto.

The invention, together with further objects and advantages thereof,will best be understood by reference to the following description takenin connection with the accompanying drawing, and its scope will bepointed out in the appended claims.

In the drawing, in which like parts are indicated by like referencenumerals:

Figure 1 is a diagram illustrating one embodiment of the invention asapplied to a variable impedance device for lse with an alternatingvoltage;

Fig. 2 is a diagram illustrating an embodiment of the invention asapplied to a resonant circuit, and

Fig. 3 is a diagram illustrating an embodiment of the invention asapplied to an oscillator.

With particular reference to Fig. l, the invention is shown embodied ina circuit including a pair of input terminals 11 and 12, a seriesarrangement therebetween of a fixed and stable capacitance condenser 13and a rotor coil 14 mounted on the rotor 14' of a rotary transformer 15,and an amplifier tube 17. The transformer is excited by voltage appliedto a stationary coil 16, the immediate source of this voltage being thetube 17 connected as a cathode follower. A cathode follower type circuitis preferably employed in the embodiment shown in Fig. 1 because it hascertain desirable characteristics which are particularly advantageous inthis application, as will subsequently be described in detail.

The circuit includes the tube 17, the plate 18 of which is connected bya conductor 19 to the positive side of a B battery 20. The negative sideof this battery is connected to a biasing resistor 21 which has acapacitor 22 arranged in parallel therewith, this condenser permittingthe flow of alternating current therethrough without appreciablyaltering the bias which is obtained by the flow of direct currentthrough the resistor 21, all as well under stood in the art.

The other end of the resistor 21 is connected by a conductor 23 to onelead of the transformer exciting coil 16, the other lead of this coilbeing connected to the cathode 24 of the electron tube 17. A circuit isthereby completed through the exciting coil 16 through the tube 17 andthe battery 20 and through the resistor 21 and capacitor 22 back to theexciting coil. A capacitor 25 may be connected in parallel with theexciting coil 16 for the purpose of creating a circuit in conjunctionwith the exciting coil 16 approximately resonant over a substantialrange of frequency.

The voltage across coil 16 is controlled by the grid 26 of the electrontube 17, this grid being connected by a conductor 27 to the terminal 11.The other terminal 12 is connected by a conductor 28 to a point 29 atthe lower point of the biasing resistor 21. The exciting coil 16 beingconnected in the cathode circuit of tube 17 makes this a cathodefollower circuit, such a circuit inherently having linear amplificationand an input im pedance which is very high with respect to the outputimpedance. A high input impedance is desirable since it is necessary toprevent drawing any current from the source of external voltage,otherwise there results a change in the apparent impedance of thedevice. Linear amplification is needed to excite the rotary transformerwith a voltage whose instantaneous magnitude is a linear function of themagnitude of the external voltage, the purpose of which willsubsequently become clear. The cathode follower circuit is also verystable, that is, the linear amplification is not materially affected byreasonable variations in the characteristics of the tube.

It will now be apparent that an external alternating voltage applied tothe terminals 11 and 12 will determine the character and magnitude ofthe voltage applied to the transformer exciting coil 16. Statedotherwise, the character and magnitude of the exciting coil voltage isderived from the applied voltage while putting a negligible load on thesource thereof, the amplifier tube 17 effectively isolating thetransformer 15 from the source of the voltage applied to terminals 11and 12. Some form of isolating means is, of course, needed to preventthe drawing of any appreciable current from the source of appliedvoltage by the control means.

The rotary transformer 15 is designed to operate over an unsaturatedrange, and, accordingly, for any given position of the rotor coil 14,the voltage output thereof is a linear function of the excitationvoltage of coil 16. The voltage of cathode 24 following the voltage ofthe grid 26, it follows then that for any given position of the rotorcoil 14, the output voltage thereof will be a fixed proportion of thevoltage applied to terminals 11 and 12.

The angular position of the rotor 12 may be controlled by a control knob20 which is mechanically interconnected with the rotor 14 asschematically indicated by the dash line 31. Rotation of the rotor 14'will alter the flux linkage between the exciting coil 16 and the rotorcoil 14 and will result in a change in the magnitude of the rotor coilvoltage with respect to the voltage applied to terminals 11 and 12. Whenthe plane of the rotor coil is aligned with the magnetic field of thetransformer the voltage output of the coil is zero, with the result thatthe apparent impedance of the device will be equal to the capacitivereactance of the condenser 13 plus the reactance of the rotor coil 14which is preferably made very small (negligible) with respect to thereactance of the fixed capacitance condenser 13.

As the rotor coil 14 is turned from its neutral position, a voltage isinduced therein of the same frequency as the frequency of the appliedvoltage, and this voltage can be made to buck or boost the appliedvoltage depending upon the direction in which the coil is turned. If therotor coil output is made to boost the applied voltage, a greatercurrent will flow in the condenser circuit with the result that theapparent capacitance of the circuit is increased. Similarly, if the coilis turned in the opposite direction, its output will buck the appliedvoltage with the result that the current in this circuit will bedecreased and the apparent capacitance will be less.

The rotary transformer 15 is preferably so designed that the voltageoutput of the rotor coil 14 is a linear function of the position of therotor 14' and hence of the control knob 30. This may be accomplished ina manner well known in the art by providing a uniform radial air gapbetween the pole faces and the rotor, with the magnetic field uniformlydistributed thereover, and

by arranging the small number of turns of the rotor coil in closelyspaced relationship. In place of the rotary type, other transformerdesigns may be used in which a continuously variable voltage ratio isobtained by means of movable coils or movable flux-carrying members. The

transform-er 15 serves as a variable voltage ratio device whose outputvoltage is a linear function of its excitation voltage and, preferably,a linear function of some mechanical displacement. Any device havingsuch characteristics may be used in place of the rotary transformer.

If desired, the positioning of the rotor coil 14 can be made anautomatic or semi-automatic function of some external apparatus.However, since the embodiment of the invention presently being describedconcerns only the impedance device itself, the details of such a controlcircuit will not be described herein.

The impedance of the exciting coil 16 is made very high relative to theimpedance of the tube circuit and consequently there is good voltageregulation in the tube circuit. The impedance of the rotor coil ispreferably made small with respect to that of fixed capacitor 13 so thatgood voltage regulation is obtained throughout the internal voltagegenerating system, all as well understood in the art.

The voltage amplification ratio of the cahode follower 7 tube circuit isinherently less than one. This is not objectionable in this particularapplication since the exciting coil 16 may be adapted for operation withthis somewhat smaller voltage.

The range in magnitude of the internal voltage, and, hence, the range inmagnitude of the apparent impedance are limited, for any given voltagegain in the amplifier, by the maximum usable voltage ratio of the rotarytransformer. The impedance range is centered about the impedance valueof the fixed impedance 13 since a given maximum internal voltage isarranged in either bucking or boosting relationship, and when theinternal voltage is zero, i. e., neither buck nor boost, the apparentimpedance is that of element 13. However, this impedance range can bevaried by changing the amplifier gain.

Variation in voltage amplification in a cathode follower amplifier islimited and the amplification is less than one in all cases and,accordingly, better control of this nature can be obtained by using anegative feedback amplifier such as that described below and shown in Ifdesired, an amplifier may be inserted in the circuit to amplify theoutput of the rotary transformer before it reaches the load circuit ofthe applied voltage. Such an amplifier may be made adjustable so thatthe range of the internal voltage may be varied.

The increased or decreased impedance range obtained by varying amplifiergain will still be centered about the value of the fixed impedance 13.This characteristic of the impedance device has considerable practicaladvantage in the adjustment thereof. The central or neutral value ofimpedance is determined by selection of a fixed impedance element of thedesired value. The range of apparent impedance for any given amplifiergain can then be determined by rotating the rotor of the transformerthrough its maximum desired angular displacement. If the variation inapparent impedance is then found to be too great or too small, theamplifier gain can be adjusted by any suitable means with the knowledgethat any adjustment will result in a range of apparent impeilancecentered about the original central or neutral va ue.

This variable impedance device may have incorporated therein, in placeof the condenser 13, an inductance coil 13a of constant and stable valuesuch as one obtainable by an iron core with an air gap, or a resistor13b of constant and stable value, both of which are shown in phantomlines in Fig. 1. In each of these cases, the operation of the deviceremains the same, the frequency of the rotor coil output always beingthe same as that of the voltage applied to terminals 11 and 12 and inthe proper phase relationship therewith, and the voltage magnitudealways being a linear function of the magnitude of the applied voltageand of the position of the control knob 38.

Thus it is seen that the variable impedance device comprises simply afixed impedance element and a voltage source arranged in seriestherewith, the output voltage of that source deriving its character andmagnitude from the voltage applied to the device without burdening thelatter, and the magnitude of the internal voltage being proportional to,and a linear function of, the magnitude of the applied voltage. Thelatter proportion is made adjustable, as by turning the rotor coil 14,in order that the apparent impedance be selectably or automaticallyadjustable.

The rotary transformer 15 employed in the embodiment of the inventiondescribed above is well known as a variable inductance device of itself,i. e., as a variometer. However, when used in that manner the inductanceis not stable, i. e., the variable permeability of the magnetic circuitcauses an undesired change in inductance when the voltage appliedthereto is altered in frequency or magnitude. When used in this fashion,the coil 16 may be connected in series with coil 14 and the two windingsconnected to the voltage source. The inductance then presented to thevoltage source is determined by the position of coil 14 whose flux addsto, or subtracts from, that due to coil 16. So long as there is nochange in the fiux of the core, that is, so long as the applied voltagedoes not change, and the air gap does not change, etc., the value ofinductance may be sensibly constant. But when the voltage changes, orthe air gap changes, the flux through the core changes and hence theinductance changes. In other words, the permeability of the iron core isa function of the voltage, air gap, etc., inductance being a function ofpermeability, when permeability changes the inductance changes.

When the rotary transformer is used as in the circuit shown in Fig. l,the device is employed as a transformer or a ratio device rather than aninductance device and its variable perm ability has a negligible effect.That is, the voltage induced into coil 14, for any one position, isalways equal to the voltage applied to coil 16 multiplied by a constantwhich is the turn ratio of coils 14 and 16 and a function of theposition of coil 14.

if the voltage applied to coil 16 changes and the fiux through the core(and thus the permeability) changes, the voltage of coil 14, for thesame position, is still the same proportion of the voltage of coil 16.The linearity of output to input voltage has been maintained. If the airgap should expand due to temperature, etc., so that the total flux dropsa certain percentage, for example, ten percent, the voltage of coil 14would not change so long as the voltage of coil 16 has not changedbecause the air gap fiux pattern has not changed. Again the linearity ofinput to output voltage has been maintained.

in other words, according to the invention, the transformer ratio of therotary transformer is used which is a function of the transformerconfiguration and which is independent of permeability to obtain alinear and stable element, whereas the same rotary transformer used asan inductance device has inductance proportional to permeability andthus is non-linear and unstable.

Moreover, at audio and lower frequencies, the rotary transformerrepresents the most satisfactory instrument to produce smoothreproducible electrical characteristics. Since the rotor may be mountedon low-friction bearings, the moving force required is small compared toslide wire reactors which have high moving friction. The rotarytransformer may be made quite small relative to a variable condenser ofcorresponding reactance.

Attention is now directed to Fig. 2 in which another embodiment of theinvention is shown applied to a resonant circuit, its function being tovary the resonant frequency of such a circuit and to maintain suchresonant frequency constant for any given settting of the device eventhough the external voltage applied to the circuit varies in magnitude.The terminals 11 and 12 are connected, as shown, to a circuit 41consisting of an inductance coil 42, a condenser 4-3, and the rotor coil14 arranged in series with the condenser 43. When the voltage output ofthe rotor coil is zero, the resonant frequency of the circuit 41 will bedetermined by the fixed reactances of the inductance coil 42 and thecondenser since the reactance of the rotor coil 14 is of such smallmagnitude that it may be disregarded. Coil 42 and condenser 43 arestable elements, as already indicated.

If, however, the rotor coil 14 is made to produce a voltage of the samefrequency as that applied to the terminals 11 and 12, the chargingcurrent in the condenser circuit will be made greater or lesser thannormal depending upon whether the rotor coil voltage is in boosting orbucking relationship with the applied voltage. 1f the charging currentis made greater, as by arranging the rotor coil voltage in boostingrelationship, the apparent capacitance is increased and the resonantfrequency of the resonant circiut 41 is lowered, as is readilyunderstood by reference to the Well known formula:

in which fr is the resonant frequency, 11' is 3.1416, L is inductance inhenries, and C is capacitance in farads. Similarly, if the voltageoutput of the rotor coil 14 is made to buck the applied voltage, theapparent capacitance is decreased, and the resonant frequency willincrease.

The rotary transformer 15 may be identical to that described in Fig. l,a control knob 30 again being provided to control the position of therotor coil 14. Excitation voltage is supplied to the stationary coil 16through a negative feedback electron tube amplifier circuit. The tube 44has a plate 45 which is connected to the transformer exciting coil 16through a conductor 46. The other terminal of the exciting coil isconnected through a conductor 47 to the positive terminal of a B battery48. The negative terminal of this battery is connected through aresistor 49 to the cathode i) of the electron tube 44, a condenser 51being arranged in parallel with the resistor 49. A condenser 25 mayagain be placed in parallel with the transformer exciting coil 16 toestablish a substantially resonant load circuit. The plate current ofthe tube 44, and, hence, the excitation current, is controlled by thegrid 51 which is connected through a resistor 52 to the terminal 11, theother terminal 12 being connected by a conductor 53 to a point 54 in thetube circuit.

In order that the voltage amplification factor of the amplifier beconstant, which is necessary to preserve a linear relationship betweenthe signal input and the voltage output, the amplifier is connected as anegative feedback amplifier. That is, plate 45 is connected to grid 51through the condenser 55 and the resistor 56. With linear amplificationthus obtained in the tube circuit, the voltage output of the rotor coil14 will be a linear function of the applied voltage for the reasonsstated above in describing the apparatus of Fig. 1. In this circuit alsothe input impedance is high so the transformer is effectively isolatedfrom the resonant circuit.

Accordingly, when an alternating voltage of a certain frequency andmagniude is applied to the circuit 41 through the terminals 11 and 12,the rotor coil 14 will produce a voltage whose character and magnitudeare derived from the applied voltage. That is, the frequencies will beequal and the voltages will either be in phase or 180 de grees out ofphase depending upon the position of the rotor coil 14. Also, themagnitude of the internal voltage will be a linear function of themagnitude of the applied voltage, the proportions being selectablyadjustable by means of the control knob 30.

This internal voltage being in series with the condenser 43 will add toor subtract from the external voltage as applied to the condenser. Thischanges the effective capacitance of the condenser side of the tunedcircuit, and, consequently, changes the resonant frequency of thecircuit 41. As the magnitude of the applied voltage is increased ordecreased, the internal voltage varies in direct proportion therebymaintaining a fixed apparent capacitance and a fixed resonant frequency.The ratio between the magnitudes of the internal and external voltagescan be selectively adjusted by control of the position of the rotor coil14 whereby the effective or apparent capacity and the resonant frequencyare similarly adjusted.

The internal voltage may be placed in series with the inductance coil 42instead of the condenser 43. The internal voltage will then vary theeffective inductance of the coil and, consequently, the resonantfrequency of the circuit.

In Fig. l, a cathode follower is employed and certain advantages thereofhave geen discussed. In the circuit of Fig. 2, a negative feedbackamplifier is employed to obtain linear amplification. Either of theseamplifiers for energizing the primary or exciting coil 16 of the rotarytransformer may be used in either type of circuit. Other forms ofamplifiers may be used so long as the amplification factor is constantand the input impedance is high so as to effectively isolate the voltagecoil 14 from the source of the voltage at terminals 11 and 12.

While inductance coil 42, condenser 43, and coil 14 are shown connectedas a parallel resonant circuit, With voltage applied across the circuit,it will be understood that the applied voltage in coil 42 and condenser43 may be arranged in a series circuit. Moreover, it is pointed out thatwhile inductance 42 and condenser 43 of Fig. 2 are disclosed as forminga circuit whose resonant frequency may be varied by means of changingthe magnitude of an impedance, it is equally applicable to producing avariation in the phase angle of an impedance.

The resonant circuit shown in Fig. 2 is readily adapted to use as thefrequency determining portion of a variable frequency oscillator and isso shown in Fig. 3 where, however, a third form of isolating amplifieris shown which will be described briefly.

The isolating amplifier 75 shown in Fig. 3 is well known in the art andis connected in What is commonly referred to as a cathode degenerationcircuit. The resistance 76 is of a substantial value and serves toreduce the variation in potential of the grid 77 relative to the cathode81. The degeneration thus obtained eliminates the necessity of thenegative feedback circuit shown in Fig. 2, while still maintaininglinear amplification.

The resonant circuit 41 is connected as shown with the internal voltagegenerated by rotor coil 14 being in series with the inductance coil 42,this having been suggested as an alternative arrangement in thedescription above of the apparatus shown in Fig. 2. The terminal 12 isconnected to ground as shown while the terminal 11 is connected to grid77 and through a resistor 61 and a coupling condenser 62 to the plate 63of an amplifier tube 64. The plate 63 of tube 64 is connected through aresistor 65 to a source of positive potential, such as a B battery whilethe cathode 66 of the tube is connected through a resistor 67 to ground.

The plate 73 of the tube 75, in addition to being connected to theexciting coil 16 of the rotary transformer 15, as in Fig. 2, is alsoconnected through a conductor 79, a condenser 68 and a resistor 69 t0the grid 70 of the tube 64, a grid leak resistor 71 being connected asshown. The entire circuit oscillates at a frequency governed by theresonant circuit 41, the in-phase feedback voltage, neces sary toproduce sustained oscillations, passing from plate 63 of amplifier 64 tothe grid 77 of the tube 75 through the condenser 62 and the resistor 61.

In oscillators, it is necessary to have an amplifier with an inputcircuit to which a portion of the energy from the amplified outputnecessary to sustain the oscillations is fed in proper phase. Since theamplifier now reamplifies the fed back portion of energy and feeds backa portion thereof, there is a progressive build-up of the amplitude ofthe oscillations until, in a properly designed amplifier, some elementof the circuit becomes non-linear of otherwise changes its manner ofoperation. At this point the percentage of energy feedback is decreasedand further amplification ceases. In such an oscillator the amplitude ofthe oscillations is maintained at a desired value. If no reduction inthe percent of feedback occurs, the circuit would generate sufficientcurrent to destroy itself. In general, it is not of prime importance asto what element in the oscillator circuit becomes non-linear and it maybe the amplifier tube which comes to operate on the nonlinear portion ofits characteristic curve.

In the apparatus of Fig. 3, the isolating amplifier 75 used indeveloping the internal voltage must provide linear amplification forthe reasons discussed above in connection with amplifiers 17 and 44.Accordingly, of the amplifiers the circuit of tube 64 must be thenonlinear element.

The circuit of osciilator 64 is designed, then, by proper selection ofcircuit constants, that is, the cathode, grid and plate resistors andthe plate voltage, such that the feedback amplifier 64 operates over arange of amplification in which it becomes non-linear when oscillationsof the desired amplitude are generated. At the same time, the voltagefed back by amplifier 64 to the resonant circuit 41 and the isolatingamplifier 75 is limited to a value which permits linear amplification bythe isolating amplifier. Hence, a fundamental characteristic of theoscillator of Fig. 3 is the combination of the linear amplifier 75 fordeveloping the internal voltage with the nonlinear, feedback amplifier64. Stated otherwise, the ranges of linear operation of the twoamplifiers are so related that the feedback amplifier may operate over anon-linear range of amplification, thereby sustaining oscillation at apredetermined level, whereby the voltage fed to the isolating amplifieris of such value that the latter amplifier operates over a linear range.

The resonant frequency of the circuit 41 may be altered by changing theangular position of the rotor coil 14 through operation of the controlknob 30. This changes the internal voltage fed to the inductance side ofthe resonant circuit which in turn changes the apparent inductance andthe resonant frequency of the resonant circuit 41.

The output voltage of the oscillator may be tapped off at any one ofseveral points, but is preferably taken from a fixed secondary coil 72of the rotary transformer 15. This particular point is selected becauseit is a relatively low impedance, high power source, and, consequently,can supply a substantial amount of power to its terminals 73 and 74 withgood voltage regulation.

The oscillator circuit, shown in Fig. 3, is made to have a variablefrequency output by including therein a voltage source in series withone of the reactance elements in the resonant circuit, this voltagesource producing a voltage which is derived from the voltage applied tothe resonant circuit. In order that the frequency of the oscillator mayremain constant, the internal voltage place in series with one of thereactance elements should vary as a linear function of the applied orfeedback voltage, this being accomplished in the circuit shown in Fig. 3through the use of the linear amplifier and a generator, i. e. therotary transformer 15, whose output voltage is a linear function of itsexcitation.

In the embodiment of Fig. 1, the voltage amplification of the cathodefollower amplifier normally is less than one. Accordingly, the range ofimpedance variation is limited. This may be visualized by noting thatfor any given deflection of rotor 14a and for any given rotor coil 14,the voltage which may be supplied in series with condenser 13 is limitedby the amplification of the amplifier.

In the embodiment shown in Fig. 2, the voltage amplification of theamplifier may be made greater than one by a reasonable amount throughvarying the values of resistors 56 and 52 relative to each other, whilethe gain of the amplifier shown in Fig. 3 may be varied by altering thevalue of the resistor 76. Such gain variation permits an increase inrange of impedance or resonant frequency variation. The amplifiers ofFigs. 2 and 3 may be used with the circuit shown in Fig. 1. Thus with anincrease in amplification of a two to one ratio, for example, thevoltage available for connection in series with condenser 13, is alsoincreased by a factor of two. Consequently, with the same deflection ofcoil 14, the increase or decrease in apparent impedance has beenmagnified by a factor of two.

In the circuit of Fig. 2, an increase in the gain of the amplifierresults in an increase in the range of resonant frequency variation withgiven apparatus. The normal resonant frequency of circuit 41 isdetermined only by the inductance coil 42 and condenser 43, andpositioning of coil 14 to one side or the other of its neutral positionproduces bucking or boosting voltages which are in series with condenser43. With increased amplification the coil 14, in any given position,produces increased voltages and hence greater deviation from the normalresonant frequency. This is of advantage in the circuit of Fig. 3 sincethe frequency range of the oscillations produced is increased.

A secondary amplifier for amplifying the output of the rotor coil 14,suggested in connection with the circuit shown in Fig. 1, can also beadded, if desired, to the circuit shown in Figs. 2 and 3.

The invention has been described above as applied to a simple variableimpedance, as applied to a resonant circuit, and as applied to anoscillator, and its use has been suggested as a means of changing thephase angle of an impedance. Many other applications of the inventionmay be found which fall in the scope of the invention, the particularapplications described above being ones in which the invention performsa function peculiar to its nature and produces results previouslyunobtainable except through the use of more complex and otherwise lesssatisfactory apparatus.

While particular embodiments of the invention have been shown, it willbe understood, of course, that the invention is not limited theretosince many modifications may be made, and it is, therefore, contemplatedby the appended claims to cover any such modifications as fall withinthe true spirit and scope of the invention.

The invention having thus been described, what is claimed and desired tobe secured by Letters Patent is:

1. In an oscillator having a resonant circuit for determining the outputfrequency thereof, said resonant circuit including a capacitivereactance element and an inductive reactance element in parallelrelationship; a

nonlinear amplifier having its output connected across said resonantcircuit for sustaining oscillation therein at a predetermined level, anda linear amplifier whose output is connected in series with one of saidreactance elements, the input of said linear amplifier being connectedacross said resonant circuit whereby the magnitude of the output voltagethereof is a linear function of the magnitude of the voltage across saidresonant circuit, and the input of said nonlinear amplifier beingconnected to said liner amplifier whereby the output of said nonlinearamplifier is controlled by said linear amplifier.

2. In an oscillator having a resonant circuit for determining the outputfrequency thereof, said resonant circuit including a capacitivereactance element and an inductive reactance element in parallelrelationship; a nonlinear vacuum tube amplifier having its outputconnected across said resonant circuit for sustaining oscillationtherein at a predetermined level, and a linear vacuum tube amplifierwhose output is connected in series with one of said reactance elements,the input of said linear amplifier being connected across said resonantcircuit whereby the magnitude of the output voltage thereof is a linearfunction of the magnitude of the voltage across said resonant circuit,and the input of said nonlinear amplifier being connected to said linearamplifier whereby the output of said nonlinear amplifier is controlledby said linear amplifier.

3. In an oscillator having a resonant circuit for determining the outputfrequency thereof, said resonant circuit including a capacitivereactance element and an inductive reactance element in parallelrelationship; a nonlinear amplifier having its output connected acrosssaid resonant circuit for sustaining oscillation in said resonantcircuit at a predetermined level, a voltage ratio device having itsoutput connected in series with one of said reactance elements, saidvoltage ratio device having a linear relationship of input voltage tooutput voltage and being adjustable to vary said ratio, and a linearamplifier, the output of said linear amplifier being connected to theinput of said voltage ratio device and to the input of said nonlinearamplifier, the input of said linear amplifier being connected acrosssaid resonant circuit.

4. In an oscillator having a resonant circuit for determining the outputfrequency thereof, said resonant circuit including a capacitivereactance element and an inductive reactance element in parallelrelationship; a nonlinear amplifier having its output connected acrosssaid resonant circuit for sustaining oscillation in said resonantcircuit at a predetermined level, a rotary transformer having its outputcoil connected in series with one of said reactance elements, saidrotary transformer having a linear relationship of input voltage tooutput voltage and being adjustable to vary said ratio, and a linearamplifier, the output of said linear amplifier being connected to theinput coil of said rotary transformer and to the input of said nonlinearamplifier, the input of said linear amplifier being connected acrosssaid resonant circuit.

5. An impedance device comprising a fixed impedance element, a voltageratio device having its output arranged in series with said fixedimpedance element, said voltage ratio device having a linearrelationship of output voltage to input voltage, and an amplifiercircuit including an electron tube, the input of said voltage ratiodevice being arranged in the cathode circuit of said amplifier wherebythe magnitude of the output voltage of said amplifier is substantially alinear function of the magnitude of the input voltage, the input of saidamplifier being arranged across said series arrangement of said fixedimpedance element and the output of said voltage ratio device.

6. An impedance device comprising a fixed impedance element, a voltageratio device having its output arranged in series with said fixedimpedance element, said voltage ratio device having a linearrelationship of output voltage to input voltage, and an amplifiercircuit including an electron tube, the input of said voltage ratiodevice being arranged in the plate circuit of said amplifier, saidamplifier including negative feedback means of such character as tomaintain a linear relationship between the magnitude of the voltageoutput and the voltage input of said amplifier as the magnitude of thevoltage input varies over a substantial range, the input of saidamplifier being connected across said series arrangement of said fixedimpedance element and the output of said voltage ratio device.

7. An impedance device comprising a fixed impedance element, a variablevoltage ratio device having its output arranged in series with saidfixed impedance element and having a negligible impedance relative tosaid fixed impedance, said voltage ratio device having a substantiallylinear relationship of output voltage to input voltage, an amplifiercircuit, the input of said voltage ratio device comprising the output ofsaid amplifier circuit, and means in said amplifier circuit to maintainthe magnitude of the output voltage of said amplifier substantially as alinear function of the magnitude of the input voltage, the input of saidamplifier being arranged across said series arrangement of said fixedimpedance element and the output of said voltage ratio device.

8. Apparatus for presenting a resonant circuit of adjustable resonantfrequency to an external alternating voltage, said apparatus comprisingan inductive reactance element, a capacitive reactance element, avariable voltage ratio device having its output arranged in series withone of said reactance elements, said output having a negligibleimpedance relative to said reactance elements, said output and saidlast-named reactance element being arranged in parallel with the otherof said reactance elements, an amplifier circuit, and means in saidamplifier References Cited in the file of this patent UNITED STATESPATENTS 1,113,149 Armstrong Oct. 6, 1914 1,783,557 Bethenod Dec. 2, 19301,779,382 Mathes Oct. 21, 1930 1,946,047 Van der Pol et al Dec. 6, 19341,997,407 Hofer Apr. 9, 1935 2,071,564 McNicolson Feb. 23, 19372,140,339 Travis Dec. 13, 1938 2,155,404 Craft Apr. 25, 1939 2,220,770Mayer Nov. 5, 1940 2,379,689 Crosby July 3, 1945 2,406,125 Ziegler Aug.20, 1946 2,459,842 Royden Jan. 25, 1949

